The present invention relates to a method of producing coated paper or paperboard. In addition, the present invention relates to a method of applying coating compositions having a high viscosity under conditions of high shear to substrates.
In the manufacture of printing paper pigmented coating compositions, which have a considerably higher solids content and viscosity than photographic solutions or emulsions, typically are applied, for example, by blade type, bar (rod) type or reverse-roll (film) type coating methods at high line speeds of above 1000 m/min. Any or all of these methods are commonly employed to sequentially apply pigmented coatings to a moving paper or paperboard surface.
However, each of these application methods inherently has its own set of problems that can result in an inferior coated surface quality. In the case of the blade type coating method, the lodgment of particles under the blade can result in streaks in the coating layer, which lowers the quality of the coated paper or paperboard. In addition, the high pressure that must be applied to the blade to achieve the desired coating weight places a very high stress on the substrate and can result in breakage of the substrate web, resulting in lowered production efficiency. Moreover, since the pigmented coatings are highly abrasive, the blade must be replaced regularly in order to maintain the evenness of the coated surface. Also, the distribution of the coating on the surface of the paper or paperboard substrate is affected by the surface irregularities of the substrate. An uneven distribution of coating across the paper or paperboard surface can result in a dappled or mottled surface appearance that can lead to an inferior printing result.
The bar (rod) type coating method is limited as to the solids content and viscosity of the pigmented coating color that is to be applied. Pigmented coatings applied by the bar type coating method are typically lower in solids content and viscosity than pigmented coating colors applied by the blade type method. Accordingly, for the bar type coating method it is not possible to freely change the amount of coating that can be applied to the surface of the paper or paperboard substrate. Undesirable reductions in the quality of the surface of the coated paper or paperboard can result when the parameters of coating solids content, viscosity and coatweight are imbalanced. Moreover, abrasion of the bar by the pigmented coatings requires that the bar be replaced at regular intervals in order to maintain the evenness of the coated surface.
The roll type (film) coating method is a particularly complex process of applying pigmented coatings to paper and paperboard in that there is a narrow range of operating conditions related to substrate surface characteristics, substrate porosity, coating solids content, and coating viscosity that must be observed for each operating speed and each desired coatweight to be achieved. An imbalance between these variables can lead to an uneven film-split pattern on the surface of the coated paper, which can lead to an inferior printing result, or the expulsion of small droplets of coating as the sheet exits the coating nip. These droplets, if re-deposited on the sheet surface, can lead to an inferior printing result. Moreover, the maximum amount of coating that can be applied to a paper or paperboard surface in one pass using the roll type coating method is typically less than that which can be applied in one pass by the blade or bar type coating methods. This coating weight limitation is especially pronounced at high coating speeds.
A common feature of all these methods is that the amount of coating liquid applied to a paper web, which generally has an irregular surface with hills and valleys, is different depending on whether it is applied to a hill or a valley. Therefore, coating thickness, and thus ink reception properties, will vary across the surface of the coated paper resulting in irregularities in the printed image. Despite their drawbacks, these coating methods are still the dominant processes in the paper industry due to their economics, especially since very high line speeds can be achieved.
All of the aforementioned coating methods have in common that coating compositions having a very high viscosity under conditions of high shear and/or shear-thickening behavior cannot be applied to substrates because such coating compositions lead to unacceptable coating defects such as streaks in the coating layer or failure to meet target coatweights. Moreover, such coating compositions generally exhibit poor water-holding properties coupled with a low immobilization solids content. Coatings with poor water-holding properties generally cannot be coated with the aforementioned coating methods without lowering the coating solids and/or adding water-holding agents. In addition for drying efficiency it is desirable to coat at high coating solids content close to the immobilization solids content. This means that coatings with low immobilization solids and poor water-holding properties are particularly challenging to coat using the aforementioned coating processes.
On the other hand, there is the trend in the paper industry to use engineered pigments that are generally pigments having narrow particle size distributions or morphologies such as high aspect ratios, acicular shapes, or other irregular shapes as well as internal porosity such as found in calcined clay. Engineered pigments hereafter referred to as co-structured pigments, have also been developed. The term “co-structured pigment” should be interpreted in the sense that such pigment is modified by, for example, agglomerating specific particles to other specific particles; one example of these is calcium carbonate particles agglomerated onto talc particles, such a combination being thought to improve specific paper properties such as opacity, gloss and printing properties. Moreover, such pigments lead to improved mechanical properties of the paper.
When engineered pigments are added to a coating composition at a level of greater Than about 20 wt. % the composition typically has a high viscosity under conditions of high shear and/or shear-thickening behavior. This is due to the inability of the pigments to pack into efficient compact structures under conditions of high shear rate. Similar volumetric packing effects at conditions of high shear rate also occur with conventional coating formulations as the solids content approaches the immobilization point. This phenomenon makes it difficult or even impossible to coat such a coating composition on paper or paperboard using the aforementioned coating techniques. Generally speaking, as the viscosity at shear rates greater than 100,000 s−1 gets higher than 50 mPa·s, runnability issues become problematic. Coatings with a viscosity above 75 mPa·s are usually considered difficult to run and coatings with viscosity above 100 mPa·s are very difficult to run.
In addition, coatings with shear-thickening behavior are nearly impossible to run on the aforementioned equipment. Shear-thickening behavior is the phenomenon of an increasing viscosity as the shear rate is increased. The shear rate for the onset of shear thickening behavior can vary widely as well as the degree of increase in viscosity with increasing shear. Both aspects of the shear-thickening behavior are important and both aspects are very dependent on the solids content of the coating. For purposes of this invention a shear-thickening coating formulation is defined as one whose viscosity increases by at least 20% over an order of magnitude (factor of 10) change in shear rate for shear rates in excess of 1000 s−1.
For some coatings the onset and degree of shear-thickening behavior is an abrupt Transition and represents a severe form of shear-thickening (dilatant) behavior. For the purposes of this invention this behavior will be called Shear Blocking Behavior, and is defined by a coating whose viscosity increases by at least 100% in less than an order magnitude increase in shear rate as measured using the Parallel Plate Viscosity Test. The shear rate for the onset of shear-blocking behavior can vary widely and is very dependent on the solids content of the coating as well as factors such as the particle size distribution of the coating pigments.
Curtain coating is a relatively new coating technique. EP-A 517 223 and Japanese patent applications JP-94-89437, JP-93-311931, JP-93-177816, JP-93-131718, JP-92-298683, JP-92-51933, JP-91-298229, JP-90-217327, and JP-8-310110 disclose the use of curtain coating methods to apply one or more pigmented coating layers to a moving paper surface. More specifically, the prior art relates to:    (i) The curtain coating method being used to apply a single layer of pigmented coating to a basepaper substrate to produce a single-layer-pigmented coating on paper.    (ii) The curtain coating method being used to apply a single priming layer of pigmented coating to a basepaper substrate prior to the application of a single layer of pigmented topcoat applied by a blade type coating process. Thus a multilayer-pigmented coating of paper was achieved by sequential applications of pigmented coating.    (iii) The curtain coating method being used to apply a single topcoating layer of pigmented coating to a basepaper substrate that has previously been primed with a single layer of pigmented precoat that was applied by a blade or a metering roll type coating process. Thus a multilayer-pigmented paper coating was achieved by sequential applications of pigmented coating.    (iv) The curtain coating method being used to apply two single layers of specialized pigmented coating to a basepaper substrate such that the single layers were applied in consecutive processes. Thus a multilayer-pigmented coating of paper was achieved by sequential applications of pigmented coating.
The use of a curtain coating method to apply a single layer of pigmented coating to the surface of a moving web of paper, as disclosed in the prior art discussed above, is stated to offer the opportunity to produce a superior quality coated paper surface compared to that produced by conventional means. However, the sequential application of single layers of pigmented coating using curtain coating techniques is constrained by the dynamics of the curtain coating process. Specifically, lightweight coating applications can only be made at coating speeds below those currently employed by conventional coating processes because at high coating speeds the curtain becomes unstable, and this results in an inferior coated surface. Unfortunately, the application of consecutive single layers of pigmented coatings to paper or paperboard at successive coating stations, whether by any of the above coating methods, remains a capital-intensive process due to the number of coating stations required, the amount of ancillary hardware required, for example, drive units, dryers, etc., and the space that is required to house the machinery.
Coated papers and paperboards that have received a coating that contains an additive designed to impart functional properties, such as barrier properties, printability properties, adhesive properties, release properties, and optical properties such as color, brightness, opacity, gloss, etc., are described as functional products and their coatings may be referred to as functional coatings. The coating components that impart these properties may also be referred to as functional additives. Functional products include paper types such as self adhesive papers, stamp papers, wallpapers, silicone release papers, food packaging, grease-proof papers, moisture resistant papers, and saturated tape backing papers.
The curtain coating method for the simultaneous coating of multiple layers is well known and is described in U.S. Pat. Nos. 3,508,947 and 3,632,374 for applying photographic compositions to paper and plastic web. However, photographic solutions or emulsions have a low viscosity and a low solids content, and are applied at low coating speeds.
In addition to photographic applications, the simultaneous application of multiple coatings by curtain coating methods is known from the art of making pressure sensitive copying paper. For example, U.S. Pat. No. 4,230,743 discloses in one embodiment the simultaneous application of a base coating comprising microcapsules as a main component and a second layer comprising a color developer as a main component onto a travelling web. However, it is reported that the resulting paper has the same characteristics as the paper made by sequential application of the layers. Moreover, the coating composition containing the color developer is described as having a viscosity between 10 and 20 cps at 22° C.
JP-A-10-328613 discloses the simultaneous application of two coating layers onto a paper web by curtain coating to make an inkjet paper. The coating compositions applied according to the teaching of that reference are aqueous solutions with an extremely low solids content of about 8% by weight. Furthermore a thickener is added in order to obtain non-Newtonian behavior of the coating solutions. The examples in JP-A-10-328613 reveal that acceptable coating quality is only achieved at line speeds below 400 m/min. The low operation speed of the coating process is not suitable for an economic production of printing paper, especially commodity printing paper.
The aforementioned documents do not disclose that a coating composition having a high viscosity under conditions of high shear can be applied to a substrate using curtain coating technology. Nor do the aforementioned documents disclose that a coating composition having shear-thickening behavior can be applied to a substrate using curtain coating technology.
The technical problem underlying the present invention is the provision of a method of producing a coated paper or paperboard, whereby a coating composition having a high viscosity under conditions of high shear is applied to said paper or paperboard.